Electrical attenuator network



Jan. 3, 1933. J, DE FOREST 1,892,935

ELECTRICAL ATTENUATOR NETWORK Filed Aug. 10, 1929 4 Sheets-Sheet 1 o 13 A i INVENTOR M. 7i DE Fo/wssr BY p I 9 W J- M ATTORNEYS Jan. 3,, 1933. J, DE R T 1,892,935

ELECTRICAL ATTENUATOR NETWORK Filed Aug. 10, 1929 4 Sheets-Sheet 2 I I i I l I I I I i Y I IY 5 I I I v I l I ,-36 f4"? I H I I I I l I L I I I 4 L i /0 AMPLIFIER 5:

I I I INVENTOR 1.J. 0E FOREST ATTORNEYS jamv 3, 1933. J, DE FOREST 1,892,935

ELECTRICAL ATTENUATOR NETWORK Filed Aug. 10, 1929 4 Sheets-Sheet a AMPLIFIER INVENTOR M..T. DE FOREST BY MM V @M I OQWI ATTORNEYS Jan, 3 1933. M. J. DE FOREST ELECTRICAL ATTENUATOR NETWORK Filed Aug. 10, 1929 4 Sheets-Sheet 4 AMPLIFIER iNVENTOR M]. DE FURL-5T BY QM, d-zdbmfl ATTORNEY5 Patented Jan. 3, 1933 UNITED STATES PATENT OFFICE MATTHEW J. DE FOREST, OF JACKSON HEIGHTS, NEW YORK, A SSIGNOR, BY MESNE AB- SIGNMENTS, TO' UNITED RESEARCH CORPORATION, OF LONG ISLAND CITY, NEW

YORK, A CORPORATION 01' DELAWARE ELECTRICAL ATTEVUATOR NETWORK Application filed August 10, 1929. Serial No. 385,054.

This invention relates to apparatus for the phonographic recording of sound electrical-- ly, and more especially relates to improvements. in such apparatus wherein the sound waves to be recorded are intercepted or picked up by a plurality of sound translating dev1ces for microphones and conducted electrically to a common recorder element.

An object of the invention isto provide attenuating means for adjusting individually the level of energy delivered from the vari ous sound translating devices to the recorder element without disturbing the electrical impedance relations existing either between each input line and its respective attenuator, or between the output side of all attenuators and the common recording circuit connected thereto, whereby the impedances of such elements may be selected to reduce undesirable reflection effects.

The above object is accomplished by em-- shunt impedance. The impedances in series in the line and the shunt impedance may be varied conveniently by a single movable element. These impedances are varied in steps and have values, according to formula hereinafter described, whereby the impedance of each attenuator as viewed in opposite directions, remains substantially constant for various adjusted positions of the movable member. The relation of the impedances of the various elements is so chosen that the impedance at the input side of each attenuator substantially matches the impedance of the line with its pickup or sound translating device therein and at the same time the combined impedances at the output terminals of all the attenuators substantially matches the impedance of the common output circuit which includes the recorder.

While this is not an essential feature of the invention, the arrangement specifically described hereafter makes it possible to vary 7 the attenuation from a maximum value down to zero, thereby eliminating the dissipation of energy in theattenuator at the minimum setting such as occurs with the majority of ex sting attenuator arrangements used for this purpose.

A. further object of the invention is to reduce or prevent cross-talk between the various recording or pickup lines which are connected to the common recorder. This is accomplished with the arrangement above described by directly connecting to ether the sides of the respective lines whic have no attenuator impedance therein, whereby the corresponding line sides of the individual recording circuits are brought to substantially the same potential whereby impedance un- .balance and cross-talk between these circuits is substantially avoided.

Referring now to the drawings:

Fig. 1 shows an elementary T network, and Fig. 2'an elementary H network such as it is proposed to utilize in the attenuator arrangement.

Fig. 3 shows an electrical recording circuit utilizing two microphones each connected to the input circuit of a recording element thru variable T networks.

Fig. 4 shows a modification of Fig. 3 wherein each microphone is connected to the input circuit of the recording element thru a variable H network.

Fig. 5 shows a circuit arrangement similar to Fig. 3 but disclosing the essential structural features. of a T network attenuator adapted to vary the attenuation in successive steps by adjustment of a single control element.

Fig. 6 shows a modification of Fig. 5 wherein each attenuator is variable continuously between certain limits.

Referring to Fig. 1, the characteristic impedance Z and the current attenuation K of a T network are related to the resistance R and R in accordance with the following formulae:

That is, if such a network be interposed between the two impedances of magnitude Z, it will introduce a current attenuation equal to K therebetween. Furthermore, under such conditions the impedance looking in eitherv direction from either pair of terminals of the I great, the impedance as seen from one pair of attenuator terminals Will be Z regardless of the impedance connected across the opposite terminals thereof. It will be seen from the above formulae that by properly selecting the magnitudes of the resistances R and R any desired degree of attenuation may be obtained consistent with a preselected characteristic impedance value.

The H network of Fig. 2 is electrically the same as the T network of Fig. 1. The H netl work is obtainedfrom the T network by distributing the resistance elements R equally between the upper and lower conductors as shown; i. e., each series arm equals As a practical proposition it is desirable to utilize the T network where one side of the line is grounded, the H network being preferable where it is desired to ground the midpoint of the circuit.

1 Referring now to Fig. 3 which shows the ap lication of the T network to an electrical pic -up and recording apparatus, two p ckup microphones are shown at 1 and 2 each connected in, a local circuit containing a primary winding P or P of transformer T or' T and a direct current supply source 3 or 4. The secondary winding S of transformer T is connected to a pair of terminals 5 of a variable T network 6, while the secondary winding S of transformer T is connected to a pair of terminals 7 of the variable T network 8. The output impedances of attenuators 6 and 8, respectively, are arranged in a closed series circuit which-includes the primary winding P of transformer P by suit-' able connections extending between the pairs of terminals 9 and 35 and the winding'P The T networks 6 and 8 are shown in their generalized form each consisting of the variable series armsA and B and the shunt arm 0 interconnected by means of the sliding contacts D for adjusting the magnitudes of the arms. With the arrangement as shown each arm must be adjusted individually to its proper value although they may move in unison as shown in Fig. 5. The arms are so adjustedin accordance withEqua tions (1) and (2) as toobtain the desiredattenuation while at the same time maintaining the characteristic impedance of the network at Z which is the impedance looking into the microphone circuits from the secondary transformer windings S and S respectively. Thus each microphone, 1 or 2, works into its proper impedance. Also'by making the impedance looking into the primary winding P of transformer T connectedto the recording circuit equal to 2Z, the combined impedances of the output circuits of the T networks 6 and 8 are connected to their proper impedance.

. The secondary winding S of transformer T is connected to an amplifier 15, the output circuitof which is connected to the recordingelement comprising the electromagnet 10 operating on the pivoted armature 11 having associated therewith the cutting stylus 12 resting in operative relation upon a record 13 which in turnis suitably supported on a turntable in the well known manner for recording purposes. v

The level of energy delivered by microphone 2 to the amplifier 15 may be adjusted to a desired intensity by proper setting of the network 8. Similarly, the energy delivered by microphone 1 to the amplifier may be adjusted independently of that delivered thereto by microphone 2 by a proper setting of the T network 6. Furthermore, as will be seen, the adjustment of the energy levels delivered by the various microphones is accom- 'plished without disturbing the impedance relations existing between the various circuit elements. When the networks are set of zero attenuation, the series resistances A and B are short-circuited and the shunt arm C is on open circuit with the result that no attenuation is introduced in the circuit. This arrangement is in contrast with the methods of attenuation mentioned above wherein considerable energy is dissipated at the minimum attenuation setting. 1

Any number of sound receivers or microphones may be connected as indicated in Fig.

3 to a common input circuit. A separate attenuator is interposed between each microphone and the common input circuit for providing individual adjustment of the current delivered by the varlous microphones. The connection of the plurality of microphone circuits to the common input circuit may be accomplished in any desired manner, i. e., all the microphone circuits may be connected in series to the common input circuit, or all may be connected in parallel thereto or a series-parallel arrangement may be utilized. In any case the impedance looking into the common input circuit should be made equal to the resultant impedance looking toward the microphones in order to avoid reflection losses;

It will be noted that each of the T networks 6 and 8 has no impedance in one side of theirrespective lines, i. e., between the right hand terminals 5 and 9 also between the left hand terminals 7 and 35, and that such line sides are directly connected together by the line connecting the right hand terminal 9 to the left hand terminal 35. This brings the corresponding low impedance line sides H networks 14 and 36 replace the T networks 6 and 8, respectively, of Fig. 3. The H networks comprise the equal series arms Y, E,

F and G, and the shunt arm H. For a dey sired attenuation and a given characteristic impedance, the various arms are adjusted in accordance with the relations expressed above in equations (2) and (3). The H networks are shown in Fig. 4 in generalized form in that each series arm and the shunt arm must be individually adjusted to the desired value in each instance.

The attenuators disclosed in Figs. 3 and 4, while embodying the principles of the present invention, would not provide a very practical arrangement for adjusting the currents delivered bythe various microphones since a number of careful adjustments would be required on each attenuator to obtain a desired attenuation value. Fig. 5 shows a circuit arrangement similar electrically to Fig.

3, but utilizing T network attenuators of efficient construction in that but a single adjustment is required for obtaining a selected attenuation value. The attenuators of Fig. 5 are arranged to vary the attenuation in successive steps, the change in attenuation between the successive steps being selected to meet the needs of a particular case. Figs. 5 and 6 illustrate an alternative connection with the low impedance side of one line connected to the high impedance side of the other line. The preferred connection is shown in Fig. 3 wherein the low impedance line sides are directly connected together as above described.

The attenuator 16 is interposed between the microphone 1 and transformer T while attenuator 17 is similarly interposed between microphone 2 and transformer T the transformers serving to adjust the apparent im pedance. Each attenuator comprises a movable element 18 having rigidly atfixed thereto three conductively connected arms 19, 20 and 21, each arm being adapted to pass in succession over corresponding groups of contacts 22, 23 and 24 as the adjusting knob 25 is turned. Resistances L, M, N and O are connected respectively between the successive contacts of group 23 and one of the input terminals 5, and similar resistances L, M, N and O are connected between the successive contacts of group 22 and one of the output terminals 9. Resistances R, S, T and U are connected between the successive terminals of contact group 24 and the remaining input terminal 5 and output terminal 9. The contact arms 19 to 21, inclusive, are so positioned relative to their groups of contacts that the various arms make contact simultaneously with corresponding contacts of the respective groups 22 to 24, inclusive.

It will be seen that with .the arrangement described a T network will be connected between the input and output pairs of terminals of an attenuator for each setting of the movable element 18, the resistances L, M, N and O constituting the successive pairs of series arms and resistances R, S, T and U the corresponding shunt arms. The resistances of the series arms increase in the order L, M, N, 0, while the resistances in the shunt arm decrease in the order R, S, T, U, being suitably proportioned to maintain the characteristic impedance of the attenuator constant, while providing successively increasing steps of attenuation as the movable element 18 is rotated in a clockwise direction.

{The attenuators are designed to vary the attenuation from zero to a maximum value. Thus with attenuator 16 in the position shown, zero attenuation is introduced due to the fact that the resistance of the series arms L is zero, and the resistance of the corresponding shunt arm R is infinity since the circuit thru the shunt arm is indicated as being open. The movable element 18 of the attenuator 17 is shown in position for introducing maximum attenuation since the contact arms are on the top step connecting the large resistances O in the series arms and the small resistance U in the shunt arm.

The stops 26 and 27 are provided to set the limits of rotation for the movable element 1 18. The successive contacts of the groups 22 to 24 may be-numbered consecutively and a chart furnished showing the amount of attenuation introduced for each step, or the amount of attenuation in suitable units, such as TU may be marked directly opposite the successive contacts. The resistances L, M, N and O are, of course, proportioned in accordance with equation (1) above, While the resistances R, S, T and U are proportioned in accordance with equation (2), the desired degree of attenuation K for each step having been previously determined upon.

Attenuators of the type indicated at 16' and 17 are, of course, constructed rigorous ly in accordancewith equations (1) and The disadvantage of these attenuators, however, lies in the fact that the attenuation cannotbe varied continuously between certain limits but must be varied in successive steps. a

Fig. 6' shows a modification of the circuit of Fig. 5 wherein the attenuators shown within the rectangles 28 and 29, respectively, are adapted to vary the attenuation continuously between certain limits. The advantages of the continuously variable attenuation, however, is obtained at the expense of some loss of accuracy and in the limitation upon the a range of attenuation values available as will network type and each comprises three slide wire rheostats I, J and K covering equal sectors of a circle. The movable element 18 is provided with a knob 25 for turning the same p and has rigidly afiixed thereto three sliding contacts 19, 20 and 21 set respectively at 120 to each other, each sliding contact being adapted to pass over a different one of the slide Wire rheostats. The rheostats I and J constitute the series arms,-while K consti-s tutes the shunt armof the attenuator. This is easily observable from the fact that with the movable element 18 set at any position a circuit is traced 'from the right terminalb of attenuator 28 thru a portion of rheostat I, sliding contact 19, sliding contact 20, thru a portion of rheostat J to the right terminal 9 of the attenuator. A path" is also traced from the leftterminal 5 and the left terminal 9 thru a portion of rheostat K and the. sliding contact 20 to the midpoint between the series arms.

With theattenuators arranged inaccordance with Fig. 6, the change in resistance of the series and shunt arms is in direct "proportion to the angular setting of the movable element. Equations 1) and (2), of course, show that the shunt arm does not vary proportionally-with the series arms so that the attenuators as constructed in Fig. 6 do not produce changes in attenuation rigorously in'accordance with formulae (1) and impedance of 200 ohms, and arranged to in- As a practical proposition, however, it has been found that for an attenuator of this type constructed to vary the attenuation continuously over a relatively small range say from 2 to 20 TU, the amount of error introduced does not amount *to more than i5 per cent. In order to construct an attenuator of this type having the attenuation values between 2 and 20 TU, it is necessary to properly locate stops 26 and 27 to prevent rotation of the movable element at proper points to set the limits of desired attenuation. Thus, for the minimum attenuation limit of 2 TU, there must be a small resistance L in each i of the series arms anda large resistance K of the shunt arm between 2 'TU and 0 TU could not easily be accomplished with the slide wire arrangement shown in Fig; 6. At the position of maximum attenuation,

namely 20 TU, all of the resistances I and J should be in the series'arms with but a small amountof resistance in the shunt arm. Stop 27 is suitably located to accomplish thls result'as shown for the attenuator 28 in which the movable element 18 is shown in the position of maximum attenuation which includes the small resistance M in the shunt arm. In addition to properly locating the stops 26 and 27, the rheostat K must, of course, be properly positioned on the'circlerelative to the rheostats I and J after the fashion indicated in the drawings. r

In order to determine the amount of attenuation introduced for each settingof the v movable element 18, ascale 30 may be aflixed to the frame of the. attenuator and arm 19 equipped with a pointer 31 for indicating on the'scale 30 as shown. The scale 30 may, of course, be properly marked to indicate the attenuation in accordance With a selected scale.

In a specific case for an attenuator constructed in accordance with'Fig. 5, adapted to have a characteristic impedance of 200 ohms, and to introduce attenuations of '0, 10,20 and 30 TU, the following values were required for the various resistance elements. For the series of arms: L=0, M= 103.8, N= 163.6, and O 187.7 ohms, respectively. The corresponding values for the shunt arms were: R=open circuit, S 140.5, T=4(l.4, and U=12.7'ohms, respectively. I

For an attenuator constructed in accord ance with Fig. 6 and having a characteristic troduce an attenuation varying from 2 to 20 TU, the series rheostats J and I were given a maximum resistance of 163 ohms each and the shunt arm rheostat K a maximum resistance of 859 ohms. The stop 26 was so located and the rheostat K so positioned relative to rheostats I, and J that for the minimum, or 2 TU setting, the series arms contained 23 ohms each, While'the shunt arm contained the maximum resistance of rheostat K, namely 859 ohms. For the maximum attenuation setting of 20 ,TU, the stop 27 was positioned to include all of rheostats J and I in the series arms, namely 163 ohms each, and only a portion of rheostat K amounting to 41 ohms in the shunt arm.

While the constant impedance networks have been disclosed specifically as applied in electrical pick-up apparatus for phonograph recording, the disclosure is, of course, not to be limited in such manner, but may be used in any instance wherea plurality of energy sources are fed into a common circuit, and it is desired to provide individual adjustment of the currents. delivered from the various "sources ofsuch input circuit. A further example of a case of this sort arises in'the' radio broadcasting of an orchestral selection where a plurality of microphones are placed about the studio and the currents therefrom fed into a common amplifying circuit for transmission to the broadcasting equipment. Here again, it is desirable to provide individual adjustment of the currents from the various microphones in which case the T or H network attenuators of the type disclosed herein could be used for this purpose.

The attenuators of Figs. 5 and 6 need not be constructed specifically as shown in the drawings. For example, the rheostats I, J and K of Fig. 5 could be arranged in concentric circles of successively increasing diameters with the arms 19, and 21 properly placed one above another for tapping off desired portions of their respective rheostats.

Similarly, in Fig. 6 the contact groups 22, 23

and 24 could be arranged in concentric cir- 20 cles of increasing diameters and the contact arms placed one above another and each of suflicient length and being bent downward at its outer end to make contact with its proper group of contacts. Attenuators of the H net- 25 work type may also be constructed in accordance with the principles set forth in Figs.

5 and 6.

The T or H networks need not necessarily have equal series arms such as is required for matching equal impedances as seen from its opposite terminals. Where the impedances between which the network is connected are different, unequal series arms would be required for suitably matching the impedances.

I claim: v

1. In combination, a. plurality of sound translating devices for producing electrical waves corresponding to sound waves, lines connecting said devices respectively to a common output circuit, a variable attenuator in each of said lines, each of said attenuators comprising a plurality of impedance elements in series in the line and an im dance element in shunt thereto, each of sai attenuators having a single movable element for interposing in the corresponding line selected amounts of each impedance element for varying the energy transferred to said output circuit while substantially matchin the impedance at the in ut termmal of eac attenuator to the impedance of the line with its sound translating device therein, and while substantially matching the combined im edances at the output terminals of all sai at- 55 tenuators to the impedance of said output circuit.

. 2 In combination a plurality of sound translating devices or producing electrical .Waves corresponding to sound waves, lines 60 connecting said devices respectively to a common output circuit, a variable attenuator justed positions a T network comprising a plurality of impedances in one side of the line, substantially no impedance in the other side of the line, and an impedance in shunt to the line, and a direct connection between the respective sides of said lines in which said T networks have substantially no impedance.

3. In combination a plurality of sound translating devices 1 01' producing electrical waves corresponding to sound waves, lines connecting said devices respectively to a common output circuit, a variable attenuator in each of said lines, each of said attenuators comprising a series of T networks, a single movable element for each attenuator for 1ntroducing in any one of a plurality of adjusted positions a T network comprising a plurality of impedances in one side of the line, substantially no impedance in the other side of the line, and an impedance in shunt to the line, the values of each of said T networks being so related to the impedances of the lines connected thereto that the impedance of each T network substantially matches at the input terminal thereof the impedance of the line .with its sound translating device, while also substantially matching the combined impedances of the output terminals of said attenuators to the impedance of said output circuit connected thereto, and a direct connection between the respective sides of said lines in which said T networks have substantially no impedance.

In testimony whereof I aflix my signature.

MATTHEW J. DE FOREST.

in each of said lines, each of said attenuators comprisin a seriesof T networks, a sin' 16 movable e ement for each attenuator for 1n- 65 troducing in any one of a plurality of ad- 

